A stepping motor is a motor capable of bi-directional rotation by moving its rotor through a series of mechanically defined steps in response to the excitation of its stator windings. The number of input steps required to rotate the motor shaft through one complete revolution varies depending upon the intended application of the motor, typically having a value of 12, 24, 72, 144, 180 or 200 steps-per-revolution.
A schematic diagram of a four-phase stepping motor 100 is shown in FIG. 1. Motor 100 basically consists of a permanent magnet rotor 102 with north and south ends 104 and 106, respectively, mounted on a rotor shaft 108. A number of stator coil mounts (110, 114, 118 and 122) and stator coils (112, 116, 120 and 124) are mounted around rotor 102.
During operation of motor 100 the stator coils (112, 116, 120 and 124) are selectively energized to cause a movement in rotor 102 to a desired position via magnetic attraction between the permanent magnet rotor and the stator coils. The energization of a stator coil causes it to behave as a magnet with its strength and polarity determined by the amount and direction of current flow in the coil. Rotor 102 can change its position from the one shown in FIG. 1 to the position shown in FIG. 2, for example, by a different pattern of stator coil energization from the original pattern in FIG. 1. Energization of the stator coils is controlled by commutation logic.
The stepper motor as shown in FIG. 1 has been simplified to provide a basic general understanding of the construction and operation of stepper motors. Additional information concerning the construction and operation of stepping motors is provided in an article entitled "A Stepping Motor Primer" by Paul Giacomo. The article was published in two parts, the first part appearing in the February 1979 issue of BYTE magazine on pages 90-105 thereof, and the second part appearing in the March 1979 issue of BYTE magazine on pages 142-149 thereof.
In typical prior art brushless stepping motor systems, the commutation logic for changing the magnetic fields of the stator coils is supplied by a digital controller board. In a closed loop system the controller board receives rotor/shaft position and spin direction information from an encoder coupled to the motor shaft. In a typical two-phase shaft encoder, two output signals in the form of square waves having a 90 degree phase difference are provided. Four encoder output states or steps per motor step are thereby provided by the encoder once during each motor step, as shown in Table 1 below. The two encoder output signals are identified as signals PHASE A and PHASE B in Table 1.
TABLE 1 ______________________________________ Encoder States per Motor Step Encoder Output State PHASE A PHASE B ______________________________________ 1 0 0 2 0 1 3 1 1 4 1 0 ______________________________________
The commutation logic determines shaft position, shaft rotation and direction of rotation from the encoder output signals and signal states. The number of edges in the square waves are indicative of angular increments of rotation of the shaft and rotor; one encoder phase, i.e. the sequence of four states shown above, corresponding to one motor step. Direction of rotation can be determined from the phase relationship between the two encoder output signals. In the closed-loop system described above, positioning error for the motor shaft resulting from encoder misalignment can not exceed the size of the encoder step, i.e., 1/4 of a motor step.
FIG. 3A shows a graph of the motor torque T versus the motor shaft angle A. In general, the motor torque varies according to a sinusoidal function dependent upon the rotor position. This relationship can be expressed in equation form as follows: EQU T=k*I* sin (A) EQN 1
where T is torque, k is a torque constant, I is the applied current and A is the angular position of the rotor. FIG. 3B shows the resulting torque T when commutation changes are made at the optimum switching points, thus producing the maximum possible torque in the motor. That is, the current supplied to the coils in one state is changed at the point where the torque falls to the level of the subsequent current state (which is rising at the time, as illustrated in the figure). However, the torque shown in FIG. 3B occurs under ideal conditions.
The graph of torque versus shaft position in FIG. 3C illustrates how actual torque provided, even under optimum switching conditions, is lessened by the effects of current switching times. Torque produced is further diminished if commutation is offset from optimum switching time, e.g. a forty-five degree electrical error results in the torque waveform shown in FIG. 3D.
As can be seen in FIG. 3D, an error in alignment between the encoder and stepper motor shaft results in a loss of applied torque. It is essential that the shaft encoder be aligned with the commutation points if optimal stepper motor performance is to be achieved. Meticulous adjustment techniques must be executed to precisely and accurately align a shaft encoder to prevent the loss of applied torque shown in FIG. 3D.